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Impact Electrochemistry of Layered Transition Metal Dichalcogenides k Sofer,‡ and Martin Pumera*,† Chee Shan Lim,† Shu Min Tan,† Zdene †
Division of Chemistry & Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore and Department of Inorganic Chemistry, University of Chemistry and Technology Prague, Technická 5, 166 28 Prague 6, Czech Republic
‡
ABSTRACT Layered transition metal dichalcogenides (TMDs) exhibit paramount
importance in the electrocatalysis of the hydrogen evolution reaction. It is crucial to determine the size of the electrocatalytic particles as well as to establish their electrocatalytic activity, which occurs at the edges of these particles. Here, we show that individual TMD (MoS2, MoSe2, WS2, or WSe2; in general MX2) nanoparticles impacting an electrode surface provide well-defined current “spikes” in both the cathodic and anodic regions. These spikes originate from direct oxidation of the nanoparticles (from M4þ to M6þ) at the anodic region and from the electrocatalytic currents generated upon hydrogen evolution in the cathodic region. The positive correlation between the frequency of the impacts and the concentration of TMD nanoparticles is also demonstrated here, enabling determination of the concentration of TMD nanoparticles in colloidal form. In addition, the size of individual TMD nanoparticles can be evaluated using the charge passed during every spike. The capability of detecting both the “indirect” catalytic effect of an impacting TMD nanoparticle as well as “direct” oxidation indicates that the frequency of impacts in both the “indirect” and “direct” scenarios are comparable. This suggests that all TMD nanoparticles, which are electrochemically oxidizable (thus capable of donating electrons to electrodes), are also capable of catalyzing the hydrogen reduction reaction. KEYWORDS: catalysis . hydrogen evolution . particle coulometry
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mpact electrochemistry is a method enabling visualization of the electrochemical signal of a single nano/microparticle impacting a conductive surface.14 It is somewhat analogous to the electrochemistry of a single molecule which, upon contact with a conductive surface (electrode), can be oxidized or reduced. A single collision results in the nano/microparticle being either adsorbed onto or rebounded off the electrode surface.2 Such contact with the electrode surface may or may not lead to an electron transfer; it is largely dependent on the electrode material, applied potential, and nature of the particle. In the event of an electron transfer, there will be a transfer to/from electronic levels of the nano or microparticle instead of to/from molecular orbitals as in the case of a molecule.46 This may result in either oxidation or reduction of the particle itself, or electrocatalysis (in terms of oxidation or reduction) of the molecule/ion present in the solvent. Individual particle collision is a novel concept, which has been mainly developed by Compton and co-workers in the recent years.1 LIM ET AL.
It has been demonstrated that upon striking the electrode surface with sufficiently positive or negative potential, complete oxidation or reduction of a nanoparticle may occur.7 Subsequently, one can quantify the number of atoms involved, the size of the nanoparticle as well as the size distribution in the colloid with high precision based on the charge passed during the collision event.8 Deduction of other information such as concentration and particle type can be performed as well.4 Such particle collision technique is largely based on a method called “particle coulometry”.8,9 “Direct” impact electrochemistry, a term coined for the oxidation or reduction of the nanoparticles themselves upon striking of an electrode surface, has been widely employed to determine the size of oxidizable metal and metal oxide nanoparticles such as gold, silver, copper, and molybdenum.8,1014 Recent studies have also extended the range of detectable nanoparticles to large organic nanoparticles based on indigo, C60,15,16 as well as reducible species like Fe3O4.17 Although other instrumental techniques such VOL. XXX
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[email protected]. Received for review June 5, 2015 and accepted July 24, 2015. Published online 10.1021/acsnano.5b03357 C XXXX American Chemical Society
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Figure 1. Schematic diagrams of (A) direct and (B) indirect voltammetry of impact nanoparticles of TMD (MX2).
Figure 2. (A) Enlarged chronoamperograms of MoS2 colloids in 0.5 M sulfuric acid at (left) 0.1 V and (right) 0.75 V (vs Ag/AgCl) over a 2-s interval, and (B) summary of spike count for various particle concentrations according to (left) amplitude and (right) quantity in the first 20 s.
as AFM and TEM also provide information about the type and size of particles,18,19 the impact electrochemistry method is able to do so with high precision. This particle collision technique offers a sizable advantage in environmental monitoring, because the amount of nanoparticles released into the environment can be quantified in situ.20,21 Apart from “direct” voltammetry, there exists “indirect” voltammetry as well, which involves electrocatalysis of a surface reaction with nanoparticles. This voltammetry LIM ET AL.
was first introduced by Micka,22 before further development was demonstrated by Heyrovsky and others,2325 where polarographic and voltammetric measurements of colloidal micro- and nanoparticles result in spikes obtained in polarograms and voltammograms. Heyrovsky proposed that such spikes essentially originate from electrocatalytic currents of compounds present in the solution that have undergone catalysis following the striking of individual nanoparticles at the otherwise inert electrode surface.24 Some of VOL. XXX
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ARTICLE Figure 3. (A) Enlarged chronoamperograms of MoSe2 colloids in 0.5 M sulfuric acid at (left) 0.1 V and (right) 0.8 V (vs Ag/AgCl) over a 2-s interval, and (B) summary of spike count for various particle concentrations according to (left) amplitude and (right) quantity in the first 20 s.
the particles include MnO2, TiO2, and SnO2,2225 as well as metal powders, such as Mo, Ni, Fe, Cu, and Pt.5,22,2628 One of the common reactions electrocatalyzed by impacting particles is the hydrogen evolution reaction (HER; or proton reduction). The individual impact of a nanoparticle can catalyze and promote the reaction, which is otherwise kinetically slow at the surface of the electrode (i.e., mercury or carbon electrode). It was further demonstrated that the frequency of the impacting particles catalyzing proton reduction increases with an increase in particle concentration in the suspension.26 In this work, we wish to further explore the practicality and suitability of this technique for the detection of individual particles of various layered transition metal dichalcogenides, including MoS2, MoSe2, WS2, and WSe2. The possibility of achieving the direct “coulometric” method as well as the indirect “electrocatalytic” method using a combination of both of the fundamentally different approaches is demonstrated in this work, as illustrated in Figure 1. In this work, we also demonstrate that all MX2 nanoparticles, which are LIM ET AL.
oxidizable, are also electrocatalytically active using the dual electrocatalytic/coulometric approach. RESULTS AND DISCUSSION Here, we demonstrate that impact electrochemistry of TMDs (MoS2, MoSe2, WS2, and WSe2) can be applied using both “direct” (inherent oxidation) and “indirect” (electrocatalytic) methods. “Indirect” impact electrochemistry was first performed via hydrogen evolution reactions. Similar to previous studies, the reduction of Hþ protons in acidic electrolyte occurs when the particles strike the electrode surface, which in turn results in the generation of current responses. It is important to note that the catalytic effect of the materials is only apparent after the onset of hydrogen evolution; onset potentials are largely dependent on the nanoparticles striking the electrode surface. The onset potential value for MoS2, MoSe2, and WS2 is approximately 0.6 to 0.8 V vs Ag/AgCl (all potentials stated in this work are vs Ag/AgCl reference unless stated otherwise), as determined by previous studies, and the onset potential for WSe2 is about 1.0 V vs VOL. XXX
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ARTICLE Figure 4. (A) Enlarged chronoamperograms of WS2 colloids in 0.5 M sulfuric acid at (left) 0.1 V and (right) 0.9 V (vs Ag/AgCl) over a 2-s interval, and (B) summary of spike count for various particle concentrations according to (left) amplitude and (right) quantity in the first 20 s.
Ag/AgCl.29 These values were used to select the potential of the chronoamperometric scans. Experiments were performed at two potentials for each material of various concentrations, one less negative than the onset of HER (fixed at 0.1 V for all materials) and one more negative than the onset. Figure 2 illustrates the impacting of MoS2 nanoparticles in sulfuric acid, which results in proton reduction. An enlarged view of the activity of the MoS2 particles in a 2-s interval is shown in Figure 2A for clear observations of the impacts made. No visible HER spikes were observed at 0.1 V (vs Ag/AgCl), which is less negative than the onset potential of HER at MoS2 (Figure 2A, left); only regular background signals were detected. In addition, a change in concentration of MoS2 had negligible effect on the signals, implying that the surface reaction was not influenced by the particles at LIM ET AL.
this stage. However, when the potential was fixed at 0.75 V, a value more negative than the onset of HER, random surges in the chronoamperograms were evident (Figure 2A, right). The amplitudes of the spikes for the different concentrations were considerably larger than for those without the nanoparticles (black line) at the same potential, clearly exhibiting the electrocatalytic behavior of the MoS2 on HER. Comparison of the impacts produced by MoS2 particles of various concentrations also shows increased frequency of successful impacts with increasing concentrations, further demonstrating the electrocatalytic activity of the MoS2 particles. Full chronoamperograms over a 50-s interval are presented in the Supporting Information (Figure S1A). With the catalytic effect of MoS2 particles established, quantification of the spikes in the first 20 s was employed to further compare HER activity at various VOL. XXX
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ARTICLE Figure 5. (A) Enlarged chronoamperograms of WSe2 colloids in 0.5 M sulfuric acid at (left) 0.1 V and (right) 1.05 V (vs Ag/AgCl) over a 2-s interval, and (B) summary of spike count for various particle concentrations according to (left) amplitude and (right) quantity in the first 20 s.
catalyst concentrations. As illustrated in Figure 2B (left), the spikes were categorized according to amplitude, and only peaks at least twice the amplitude of the background signal (denoted by δ) were considered successful impacts. The average background signal was determined from the chronoamperogram obtained at 0.75 V with no MoS2 particles present (black line, 0 ppm). For each category, a rise in concentration resulted in a general increase in the number of spikes. A significant observation is that fewer spikes were observed at larger amplitudes, and these larger amplitudes might be attributed to either the simultaneous impact of several particles leading to signal superposition, or the impact of a larger particle. The number of spikes was also plotted against nanoparticle concentration, as shown in the right panel of Figure 2B. It is evident that while HER was successfully catalyzed by MoS2 nanoparticles via impact electrochemistry, the LIM ET AL.
catalytic effect is also positively correlated with the concentration of particles. Similar chronoamperometric profiles were observed when the potential of the MoSe2 particles in catalyzing HER was examined, as illustrated in Figures 3A and S1B. Regular background responses were recorded before the onset of HER, but when the potential was fixed at a more negative value from the onset potential (0.8 V), spikes were reflected in the chronoamperograms in a random manner. This is again indicative of the ability of the nanoparticles to catalyze the generation of molecular hydrogen. In contrast with MoS2, the number of spikes recorded was not monotonic with increasing MoSe2 concentration, where a maximum spike count was attained at 8000 ppm. Spike quantification was subsequently carried out to confirm that 8000 ppm is indeed the concentration providing the highest spike frequency. Evidently, the VOL. XXX
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ARTICLE Figure 6. Scanning electron microscopy (SEM) images of (A) MoS2, (B) MoSe2, (C) WS2, and (D) WSe2 at 5000, with scale bars representing 1 μm. Inset: SEM images of corresponding materials at 30 000, with scale bars of 200 nm.
spike count for 8000 ppm of MoSe2 exceeded that of other concentrations, as compared in Figure 3B. The trend of the number of spikes for different concentrations in each peak height category in the left panel of Figure 3B also remains consistent with the overall trend line shown on the right, where the number of spikes increased from 1000 ppm until a maximum was obtained at 8000 ppm, before experiencing a drop at 10 000 ppm. This occurrence can be explained by the tendency of particles to strike and then settle on the working electrode, resulting in a limited unoccupied area of the working electrode for new particles to strike. Conceivably, MoS2 and MoSe2 particles have comparable catalytic effect on HER judging from the overall spike counts recorded in Figures 2B and 3B. The only difference lies in the correlation between the concentration of particles and the frequency of collisions, which was more notable for MoS2. Further studies on the influence of nanoparticles on HER catalysis were also carried out using two other TMDs, WS2 and WSe2. Similar to the molybdenum counterparts, no significant spikes were observed at 0.1 V (Figures 4A and 5A, Figures S1C and S1D), which is less negative than the onset potential of HER. Spikes with varied amplitudes at irregular intervals were generated when the potentials were fixed at a point (0.9 V and 1.05 V for WS2 and WSe2, respectively) after the occurrence of HER. These spikes indicate the successful catalyzing of HER upon impact of the WS2 and WSe2 nanoparticles, regarding them catalytic toward proton reduction. However, the spikes produced LIM ET AL.
with the WS2 particles are obviously more sharp and distinct as compared to WSe2. A poorer catalytic performance can be suggested for the less distinguishable spikes obtained by WSe2; this is supported by previous HER experiments carried out using voltammetric studies.29 Figures 4B and 5B summarize the amplitude and amount of spikes recorded for HER on each material. No major disparities were discovered for the total spike count of the two tungsten materials, and this observation is nearly identical to that for the molybdenum materials. The amplitudes of the spikes were relatively consistent with the predicted trend for WS2, where fewer spikes were observed at higher amplitudes. In contrast, anomalies were present for the WSe2 particles, more so for the 8000 and 10 000 ppm colloids. The number of spikes observed at amplitudes more than 10 times that of the background signal was unexpectedly more than half of the total number of spikes recorded. This inconsistency is likely due to the agglomeration of the nanoparticles at higher concentrations. Performing impact electrochemistry using TMDs as catalysts for hydrogen evolution reaction affirmed that catalysis of the reaction is indeed affected by the nature, as well as the concentration of the nanoparticle applied. Analyses of the four TMDs showed conclusively that all of them possess some degree of catalytic activity toward HER. This is in line with previous studies conducted using traditional electrochemistry.3034 In addition, the general relationship between particle VOL. XXX
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concentration and spike frequency was also proven in the TMDs, with slight differences in the selenide particles where a concentration of 8000 ppm shows the best spike result. Peaks that are more distinct were also observed when proton reduction was carried out using MoS2 and WS2 particles. Finally, the trend in spike amplitude was more evident in the molybdenum species, despite more spikes recorded for HER using tungsten particles. While “indirect” voltammetry was proven relatively successful using catalytic TMD nanoparticles, earlier applications of impact electrochemistry have only been established with small-sized nanoparticles9 and have not been fully extended to particles of larger sizes. Because TMDs are larger, we wish to verify the viability of applying this concept to these particles by size determination performed via the inherent oxidation of these particles (“direct” voltammetry). The signals obtained were then evaluated to acquire LIM ET AL.
sffiffiffiffiffiffiffiffiffi 3 QMr L ¼ FFz
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Figure 7. (Top) Chronoamperograms of oxidation of TMDs in 0.5 M sulfuric acid at þ1.1 V (vs Ag/AgCl) and (bottom) enlarged version of scans over a 2-s interval.
experimental sizes of the particles, followed by comparison with the actual sizes detected using scanning electron microscopy (SEM). SEM was performed on the TMD materials and, as displayed in Figure 6, the TMDs are not completely spherical metal nanoparticles due to their layered structures. MoS2 contains platelet-like particles of various sizes; MoSe2 particles possess uneven surfaces with a small degree of particle agglomeration. WS2 particles appear to be rounder and more uniform than do the other TMDs, while WSe2 exists as long, flat particles with uneven surfaces. Additional characterization including X-ray diffraction (XRD) and energydispersive X-ray spectroscopy (EDS) were also performed to determine the phase purity and presence of impurities in the TMDS respectively, as presented in the Supporting Information. As shown in Figure S2, the exfoliated MoS2, WS2 and WSe2 are single phase compounds, while the MoSe2 detected was in the form of Mo3Se4, suggesting a deficiency in selenium. This compound is formed upon vigorous exfoliation of lithium intercalated MoSe2. Figures S3 and S4 show almost negligible amounts of impurities in the exfoliated TMDs other than carbon and oxygen which are likely attributed to the inadvertent presence of atmospheric carbon dioxide. Some of the other impurities detected include aluminum, titanium and iron; the trace amounts suggest that the impacts caused by the materials are largely dominated by the TMDs. The size of the particles is a function of the charge passed with each spike.3 By assuming a cube-like structural conformation for the TMDs, the edge length of the TMD particles that generated successful impacts can be evaluated using an equation (eq 1) slightly modified from a derivative of Faraday's law, as such:8 (1)
where Q is the amount of net charge passing through during a collision (denoted by the area under the spike recorded in the chronoamperograms minus the average background signal) and Mr is the relative molecular mass of the TMDs. F is the density of the TMDs, F is Faraday's constant (96 485 C/mol), and z is the number of electrons transferred. Here, two electrons are transferred during the oxidation, as suggested by Brookins and illustrated in the Supporting Information (Figure S5).35 A general increase in the oxidation number of the metals from þ4 to þ6 is observed. For size determination, oxidation was performed at þ1.1 V for 50 s on 5000 ppm of all four TMDs, as illustrated in Figure 7. Six apparent oxidation signals were then chosen within a 2-s interval as the sample data set for all materials, and are illustrated in Figure 8. VOL. XXX
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Figure 8. Spikes selected for size determination of the four materials under study.
The corresponding charge passed with each spike together with other relevant data required for size determination is summarized in Table S1 in the Supporting Information. The particle edge size from each spike were also evaluated in Table S1. The average particle length for MoS2, MoSe2, WS2, and WSe2 are 496, 437, 439, and 775 nm, respectively, with slight deviations due to different particle sizes. This observation is also apparent in the SEM images in Figure 6 and images of some individual particles at higher magnification in the insets. In general, particles were observed to be approximately 400600 nm in diameter. A comparison of the calculated values in Table S1 with the actual sizes obtained using SEM suggests that the determination of particle length using the spikes on the chronoamperograms is a relatively precise technique from the small discrepancies between the overall experimental and actual values. Among the four TMDs, size determination for MoS2 appears to be the most accurate, accompanied by a small standard deviation of about 50 nm. Nonetheless, there remain some deviations between the experimental and actual particle lengths because some spikes were evaluated to be the result of impact by nanoparticles larger than 600 nm. The variation of results can be attributed to the possible the coagulation of particles due to less effective separation in the electrolyte. In addition, cube-like conformation of the nanoparticles might not be the most accurate assumption for size determination. As a result, a assumption of spherical shape was further applied with a modified equation. The results are presented LIM ET AL.
in the Supporting Information (Table S2 and eq S1); the evaluated particle radii are about half that derived from the cube-like hypothesis. Because particle size is largely dependent on the shape used for size determination, it is highly possible that the layered TMDs take on other shapes including cuboid-like or platelet-like morphologies. This undoubtedly poses challenges in the accuracy of size determination. Other than particle shape, larger particles (a few μm) were also not prominent in the recorded spikes. This might be indicative of the detection range of this technique. Despite the abovementioned limitations, this technique appears to be a viable method for size determination of particles even on a scale exceeding that achieved by prior studies, suggesting an upper detection limit of about 100 nm.9 This finding serves as further testament to the high precision promised in earlier studies, although the shape and structure of the particles remain a crucial factor for consideration. Finally, we examined whether the MX2 nanoparticles can only electrocatalyze HER (without being oxidized themselves) or be oxidized directly (without electrocatalyzing HER). This investigation is significant for understanding the hydrogen evolution reaction using MX2-based electrodes. Comparisons were drawn between the spike quantification of the “direct” (oxidation) and “indirect” (proton reduction) voltammetric studies at a particle concentration of 5000 ppm for each MX2 nanoparticle material, as illustrated in Figure 9. The bi-directional reactions exhibited a general trend in which the number of nanoparticles catalyzing the HER process was comparable to the count of MX2 NPs that were directly oxidizable. VOL. XXX
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Therefore, most, if not all electrochemically oxidizable MX2 nanoparticles are also catalytically active towards HER. CONCLUSION Impact electrochemistry has been in the frontier of electrochemical studies of nanoparticles in the past decade due to its convenience and high precision. Because it challenges other expensive methods of
EXPERIMENTAL SECTION Materials. Sulfuric acid (9598%, v/v) was obtained from Sigma-Aldrich, Singapore. Bulk materials, MoS2, WS2, MoSe2, and WSe2 (
